Project Summary Overt metastases are often diagnosed years after the removal of primary breast tumors, indicating the existence of systemically disseminated tumor cells or microscopic metastases. Adjuvant therapies have been designed to eliminate these cells. Although significant advances were made, a substantial proportion of patients still develop overt metastases, accounting for over 90% of breast cancer-related deaths. How micrometastases resume aggressive outgrowth and become incurable overt metastases remains poorly understood. Our long-term goals are to elucidate the biology underlying the survival and progression of microscopic metastases and to design therapeutic strategies against these latent tumor cells. The overall objective of this project is to investigate how tissue homeostasis of the bone, the organ most frequently affected by metastatic breast cancer, dictates the fate of bone micrometastases (BMM). Bone and bone marrow comprise of several highly distinctive microenvironment niches. Dormant disseminated tumor cells (DTCs) may reside in the perivascular niche, whereas proliferative BMM were found in the osteogenic niche that exhibit features of active osteogenesis (the bone-making process). It remains elusive how cancer cells are relocated from one niche to another, and switch their fates from dormancy to outgrowth. In search for such mechanism, we observed an interesting ?migration- by-tethering? phenomenon: cancer cells can adhere to osteogenic cells such as mesenchymal stem cells (MSCs) through a dendritic spine-like structure (DSLS) that is highly pliable and elastic. Like dormant DTCs, resting MSCs also localize in perivascular niches. Turnover of bone tissues releases signals to mobilize and chemo- attracted MSCs to sites needing osteogenesis, thereby providing a possible vehicle for cancer cells to ?ride? and relocate from the perivascular niche to the osteogenic niche. The subsequent differentiation of MSCs will then fuel the development of the osteogenic niche, and directly promoting metastasis progression. These findings lead us to hypothesize that the bone turnover process may recruit both osteogenic cells and DTCs via a ?migration-by-tethering? mechanism, and foster the development of osteogenic niche to pro-mote bone colonization. We will test these hypotheses by pursuing the following specific aims. Aim 1. To molecularly dissect the ?migration-by-tethering? mechanism and determine its role in the development of the osteogenic niche and early-stage bone colonization of DTCs. Aim 2. To determine the impact of perturbations of bone turnover on bone metastasis, and assess how this impact is mediated by the ?migration-by-tethering? mechanism that recruits DTCs to the osteogenic niche. The proposed research will have impact at multiple levels. At a cellular level, it will elucidate how cell migration can occur with assistance of microenvironment cells but without acquisition of cancer-intrinsic migratory traits. At a physiological and pathological level, it will establish connections between bone homeostasis and bone metastasis, reveal etiology of late-onset bone recurrences, and provide therapeutic insights.